The invention relates to a method of bending glass, in which method glass is heated in a bending furnace, and the glass is transferred onto a movable mould after its heating point or zone, whereby the glass bends upon the mould, and when the glass is being transferred onto the mould, the mould is moved in such a way that the horizontal velocity of at least the front edge of the mould is arranged to be substantially as high as the horizontal velocity of the glass.
Further, the invention relates to an apparatus for bending glass which apparatus comprises a bending furnace comprising heating means for heating glass; a mould for bending glass; means for transferring glass onto a mould, whereby the mould is arranged to be moved into a position following the heating point or zone of the glass; and a control unit for guiding the movements of the mould and the glass in such a way that when the glass is being transferred onto the mould, the horizontal velocity of at least the front edge of the mould is arranged to be substantially as high as the horizontal velocity of the glass.
Basically, two kinds of solutions have been suggested for bending glass, i.e. the glass is bent either inside or outside a bending furnace. Bending glass outside a bending furnace is disclosed in WO publication 95/11202 and FI publications 891646 and 863826, for example. Glass is typically heated in conventional roller furnaces. The actual bending takes place outside the furnace, usually either gravitationally or by pressing the glass against a desired form surface. When the glass is bent outside the furnace, the problem is that the glass cools during the bending. The surfaces of the glass naturally cool faster than the inside of the glass. Thus, the problem is that the bending resistance of the glass increases greatly, and in addition, the temperature of the glass will fall below the tempering temperature if the glass is taken out of the furnace only slightly warmer than the tempering temperature. Bending outside the furnace always requires additional heating of the glass into a temperature exceeding the normal tempering temperature. Overheating of the glass always leads to great quality problems in the glass, and thus a high quality level cannot be achieved for the glass. Further, as to the bending form, it is very difficult to produce flawless glass.
If glass is bent inside the furnace, a great deal of know-how is required when the glass to be heated is transferred to the bending mould. For instance, difficulties are caused by the fact that the transfer has to take place in a hot state, i.e. the temperature of the furnace has to be approximately 700xc2x0 C., for example. For example the positioning of the glass into the correct location is difficult in a hot state. Moreover, the upper surface of the glass may in some cases be coated or painted, and thus prone to damage. Further, due to its temperature, the glass is soft, and touching easily leaves traces on it. However, what it comes to optical quality, bending glass inside the furnace allows best possible glass to be produced.
In a method in which the heating and bending of the glass are performed inside the furnace, the glass is heated upon ceramic rollers. After this, the glass is lifted up from the ceramic roller arrangement by means of a vacuum lifter, transferred from the vicinity of the rollers and dropped onto an edge mould brought under the vacuum lifter. The glass is then immediately transferred to temper cooling. Hereby, however, the central area of the glass easily bends too much, in other words an xe2x80x98overbendxe2x80x99 is formed in the glass, which may even be desirable in the manufacture of vehicular glasses. However, the method is hereby not applicable to the manufacture of furniture glasses. Further, vacuum lifters leave marks on the upper surface of the glass to be bent, in particular in the treatment of coated and painted glasses.
FI publications 884443 and 931828 disclose a solution in which the glass is positioned directly upon a bending mould, whereby the glass is heated as from the initial situation in such a way that it is supported by the mould. The large number of moulds and the consequently impaired repeatability accuracy are drawbacks of this solution. Further, the manufacture of the mould itself is a problem, because it is very difficult to produce an optimal mould when the glass is supported by the mould during the entire heating and temper cooling process. Also in this method, the central area of the glass bends too much, in other words an overbend is formed in the glass.
Furthermore, a method is known which utilizes a roll arrangement which bends according to the form of the glass inside the furnace. From the roll arrangement the glass is correspondingly transferred to the roll arrangement of the cooling unit. The method is very complex and expensive, and in addition, the range of potential forms for the glasses to be manufactured is rather limited.
FI publication 82438 discloses a method of transferring glass sheets from a conveyor onto a mould. A small angle is formed between the path of the mould and the path of movement of the glass sheets exiting from the conveyor. When the glass is being transferred onto the mould, the velocity of the mould is substantially as high as the velocity of the glass. However, the transfer of the glass onto the mould is rather clumsy, and because the glass is rather soft at the moment of the transfer, there is a risk that the glass will get damaged.
An object of this invention is to provide a method and an apparatus by means of which a good result is achieved in the bending of glass.
The method according to the invention is characterized in that while glass is being transferred onto a mould, the front edge and the rear edge of the mould are arranged to have different paths of movement.
Further, the apparatus according to the invention is characterized in that while glass is being transferred onto a mould, the front edge and the rear edge of the mould are arranged to have different paths of movement.
An essential idea of the invention is that glass is heated in a furnace and transferred thereafter onto a mould following the heating point or zone in such a way that while the glass is being transferred, the horizontal velocity of the front edge of the mould is substantially as high as the horizontal velocity of the glass, and the front edge and the rear edge of the mould have different paths of movement. The idea of a preferred embodiment is that the glass is bent in a furnace. The idea of a second preferred embodiment is that the glass is heated upon rollers. The idea of a third preferred embodiment is that the front edge of the mould is slightly below the upper surface of the roller arrangement. The idea of a fourth preferred embodiment is that the mould is in an oblique position in such a way that its front edge is higher than the rear edge when the front edge of the glass passes the last roller. The idea of a fifth preferred embodiment is that the rear edge of the mould is arranged at an early stage of the transfer below the roller arrangement. The idea of a sixth preferred embodiment is that the front edge of the glass is supported after the front edge of the glass has passed the last roller, until the front edge of the glass reaches the point where it is supported by the mould. The idea of a seventh preferred embodiment of the invention is that the mould is at at least one point bent substantially in its entirety when the mould is being transferred from the position it had during the initial heating of the glass into a position where the glass is arranged upon the mould.
An advantage of the invention is that good optical quality and a flawless bending form are achieved for the glass. No marks are left on the upper surface of the glass. Further, the mould is simple, and the solution in its entirety is simple and reliable. The invention allows manufacture of bent glass in a flexible manner for different purposes, such as for automotive industry, and also for furniture, shipping and/or building industries. When it is bent inside the furnace, the glass does not have to be heated into a temperature substantially higher than the tempering temperature.